The area required is calculated per UG-37(c). For all vessel types under external pressure and for flat heads, this value is multiplied by 0.5. The required areas are calculated per Fig. UG-37.1.
The software uses dl - d, (Diameter limit minus inside hole radius) in the calculations
for the area available in the shell. This is because the code incorrectly assumes
that dl-d is always equal to d, which is only true when the natural diameter limit
is used. Because you can enter a reduced diameter limit, the software does not use
the pure code equation.